Oilfield operators are faced with the challenge of maximizing hydrocarbon recovery within a given budget and timeframe. While they perform as much logging and surveying as feasible before and during the drilling and completion of production and, in some cases, injection wells, the information gathering process does not end there. It is desirable for the operators to track the movement of fluids in and around the reservoirs, as this information enables them to adjust the distribution and rates of production among the producing and/or injection wells to avoid premature water breakthroughs and other obstacles to efficient and profitable operation. Moreover, such information gather further enables the operators to better evaluate treatment and secondary recovery strategies for enhanced hydrocarbon recoveries.
The fluid saturating the formation pore space is often measured in terms of a hydrocarbon fraction and a water fraction. Due to the solubility and mobility of ions in water, the water fraction lends itself to indirect measurement via a determination of formation resistivity. The ability to remotely determine and monitor formation resistivity is of direct relevance to long term reservoir monitoring, particularly for enhanced oil recovery (EOR) operations with water flooding and/or CO2 injection. Hence, a number of systems have been proposed for performing such remote formation resistivity monitoring.
One such proposed system employs “electrical resistivity tomography”. Such systems employ galvanic electrodes which suffer from variable and generally degrading contact resistance with the formation due to electrochemical degradation of the electrode. This variability directly affects data quality and survey repeatability. See, e.g., J. Deceuster, O. Kaufmann, and V. Van Camp, 2013, “Automated identification of changes in electrode contact properties for long-term permanent ERT monitoring experiments” Geophysics, vol. 78 (2), E79-E94. There are difficulties associated with ERT on steel casing. See, e.g., P. Bergmann, C. Schmidt-Hattenberger, D. Kiessling, C. Rucker, T. Labitzke, J. Henninges, G. Baumann, and H. Schutt, 2012, “Surface-downhole electrical resistivity tomography applied to monitoring of CO2 storage at Ketzin, Germany” Geophysics, vol. 77 (6), B253-B267. See also R. Tondel, J. Ingham, D. LaBrecque, H. Schutt, D. McCormick, R. Godfrey, J. A. Rivero, S. Dingwall, and A. Williams, 2011, “Reservoir monitoring in oil sands: Developing a permanent cross-well system” Presented at SEG Annual Meeting, San Antonio. Thus, it has been preferred for ERT systems to be deployed on insulated (e.g., fiberglass) casing. However, insulated casing is generally impractical for routine oilfield applications.
Crosswell electromagnetic (EM) tomography systems have been proposed as a non-permanent solution to reservoir monitoring. See, e.g., M. J. Wilt, D. L. Alumbaugh, H. F. Morrison, A. Becker, K. H. Lee, and M. Deszcz-Pan, 1995, “Crosswell electromagnetic tomography: System design considerations and field results” Geophysics, 60 (3), 871-885. The proposed crosswell EM tomography systems involve the wireline deployment of inductive transmitters and receivers in separate wells. However, the wells in a typical oilfield are cased with carbon steel casing, which is both highly conductive and magnetically permeable. Hence, the magnetic fields external of the casing are greatly reduced. Moreover, the casing is typically inhomogeneous, having variations in casing diameter, thickness, permeability, and conductivity, resulting from manufacturing imperfections or from variations in temperature, stress, or corrosion after emplacement. Without precise knowledge of the casing properties, it is difficult to distinguish the casing-induced magnetic field effects from formation variations. See discussion in E. Nichols, 2003, “Permanently emplaced electromagnetic system and method of measuring formation resistivity adjacent to and between wells” U.S. Pat. No. 6,534,986.
There do exist a number of apparently-speculative publications relating to permanent EM reservoir monitoring systems. See, e.g., Nichols 2003; K. M. Strack, 2003, “Integrated borehole system for reservoir detection and monitoring” US Pat. App. 2003/0038634; and A. Reiderman, L. G. Schoonover, S. M. Dutta, and M. B. Rabinovich, 2010, “Borehole transient EM system for reservoir monitoring” US Pat. App. US2010/0271030. However, it does not appear that any such systems have yet been developed for deployment, and may in fact be unsuitable for their proposed uses. For example, Nichols 2003 proposes the incorporation of long slots in the steel casing to disrupt the flow of induced counter currents in the casing, but slotted casing may be expected to have significantly weaker structural integrity and does not appear to be a viable solution. It appears that the other proposed systems fail to adequately account for the presence of casing effects, and in fact the present authors believe such effects would render the performance of these other proposed systems inadequate.